1. Field of the Invention
The present invention relates to a digital protective relay monitoring system for testing the analog input section of the relay.
2. Description of the Prior Art
One conventional system for testing the analog input section of the digital protective relay is disclosed, for example, in Japanese Patent Application Laid-open No. 53-6855. FIGS. 1 and 2 shown two forms of this system. The arrangement of FIG. 1 includes a CT 2 and a PT 3 provided on a power transmission line 1 and connected to input contacts S1 and S1' of an input switch device 8, and a mimic power transmission line 4 provided thereon with a power source 5 for testing, CT 7, and reactor 6. The CT 7 and reactor 6 are connected to input contacts S2 and S2' of the switch 8. Signals on common contacts S and S' of the switch device 8 are fed through respective A/D converters 11 and 12 to a digital processor 9.
For testing the relay, the current and voltage of the power line 1 detected by the CT 2 and PT 3 are replaced with the current and voltage of the mimic power line 4 by the switch device 8. The current and voltage on the mimic power line are produced by the power source 5, and their magnitude can be changed by the taps on the CT 7 and reactor 6 before being entered to the A/D converters 11 and 12. The current and voltage signals fed through the switch device 8 are converted into corresponding digital values by the respective A/D converters 11 and 12, and delivered to the digital processor 9. In the normal operation, the output of the digital processor 9 is sent to a trip circuit so that the power transmission line 1 is shut off by a breaker, while during the test the digital processor 9 compares the outputs of the A/D converters 11 and 12 (derived from the mimic power transmission line 4) with predetermined reference values and, if both values under comparison are not consistent, provides a signal to an alarm circuit.
Another form of the conventional digital protective relay monitoring system disclosed in the patent publication No. 53-6855 is shown in FIG. 2, in which components identical to those in FIG. 1 are referred by the common symbols. This arrangement includes memory circuits 13 and 14. A/D converters 11 and 12 are provided between a CT 2 on the power line 1 and an input contact S1 of a switch device 8 and between a PT 3 and another input contact S1' of the switch device 8, respectively. The memory circuits 13 and 14 are connected to input contacts S2 and S2' of the switch device 8, which has common contacts S and S' connected directly to a digital processor 9. Test data is stored in advance in the memory circuits 13 and 14.
For testing the performance of the relay, the common contacts S and S' of the switch device 8 are moved from the contacts S1 and S1' to the contacts S2 and S2', respectively, so that the memory circuits 13 and 14 are connected through the switch device 8 to the digital processor 9 for transferring digital data of the test current and voltage.
The conventional system arranged as described above has the following shortcomings. The system shown in FIG. 1 needs a mimic power transmission line including a power source, reactor and transformers, resulting in a large scale test equipment. During the test, the current and voltage of the actual power transmission line cannot be delivered to the digital processor, and the power line cannot be protected from a possible fault during the test in which the protective relay is out of commission. The use of the commercial power supply for the testing power source does not provide a high accuracy test result, due to a frequent voltage variation. The system shown in FIG. 2 cannot detect a fault occuring in sections in front of the A/D converters, since the test inputs are introduced at the rear of the A/D converters. In both forms of the system shown in FIGS. 1 and 2, the test is conducted at a certain interval and when a malfunctioning of the system arises the system can possibly encounter a fault of the power transmission line before detecting its own defect, and therefore the conventional system is not sufficient for monitoring the protective relay.